The present invention relates to a loop transmission line having a plurality of transmission devices which are connected to a pair of transmission loops, one being an active loop and the other, a backup loop. More particularly, the present invention is connected with a loop back system for switching the transmission loops from one to the other in response to a command sent out from one of the transmission devices.
A loop transmission system employs two independent loops which allow signals to flow therethrough in opposite directions to each other, taking into account possible failures in the transmission paths. Generally, when a failure occurs in one of the loops which is in operation, it is switched to the other loop to continue communications.
Thus, in the prior art system, major interest has been taken in failures in the transmission paths and not in the installation of additional nodes, removal of nodes, or the like.
An example of prior art systems of the type described is disclosed in "32 Mbps Optical Fiber Loop Network: H-8644", Hitachi Reviews, Vol. 31, No. 3, June 1982. In the disclosed system, when failures have occurred in both the active and backup loops, a loop service node (LSN), or master station, sends out a command for sequentially looping back remote field service nodes (FSN), or slave stations, to thereby cause the FSNs into loop back operation and, awaiting a return of a loop timing output from the LSN, locates a failure portion to set up a loop back path which excludes the failure portion. Such a system, therefore, requires the LSN to send out another command to the other LSNs in the opposite direction to loop them back, so that the whole system may be looped back. Also, each node has to be furnished with two different receiver circuits for receiving the commands, one for the active loop and the other for the backup loop.
For the above reasons, the prior art system concerned does not allow a loop to be additionally installed in or removed from the loops without temporarily cutting off the loops. This requires software or firmware countermeasures such as retry of messages at each node or momentary inhibition of transmission.